The present invention relates to an exhaust gas treating apparatus wherein exhaust gas that is discharged from municipal refuse incineration equipment, industrial waste incineration equipment, etc. is passed through a filter to remove dust and reduce the amount of harmful substances, e.g., dioxin and NO.sub.x, contained in the exhaust gas.
Hitherto, a bag filter has been used as an exhaust gas treating device for reducing the amounts of dust and dioxin contained in exhaust gas that is discharged from municipal refuse incineration equipment, industrial waste incineration equipment, etc. This bag filter comprises, for example, a suspended cylindrical filter cloth, which is used in such a way that an exhaust gas stream is passed through the cylindrical filter cloth from either the inner or outer side thereof and the purified exhaust gas is released from the other side.
In general, exhaust gas contains dust at a concentration of several tens of g/Nm.sup.3, but it is purified to a dust concentration of several tens of mg/Nm.sup.3 by passing through a bag filter. The dust collected in this way is discharged by being cleared off the cloth of the bag filter. Examples of methods of clearing the collected dust include pulse jet cleaning, back washing, shaking, etc. Among them, the pulse jet cleaming method is commonly used for exhaust gas treatment from the viewpoint of reliability and cost. The pulse jet cleaning method is carried out in such a manner that compressed air is blown into the bag filter from the purified gas side to strain and relax the filter cloth repeatedly, thereby clearing the dust from the filter cloth.
Exhaust gas from an incinerator is cooled by water spray in an exhaust gas cooler, sprayed with lime, and passed through a bag filter at 100.degree. C. to 200.degree. C., thereby capturing dioxin in the bag filter.
It is also known that dioxin can be removed by passing the exhaust gas through an activated carbon filter. In addition, composite filters comprising a carbon fiber layer and a reinforcing filter medium, which are formed together in an integral structure, are disclosed as filter dust separators for high temperature in Japanese Patent Public Disclosure No. 61-28415 (1986) and Japanese Utility Model Public Disclosure No. 63-46922 (1988). In these filters, however, a high-temperature strength filter medium is provided on the back surface of a carbon fiber layer to reinforce the fiber layer, but no bag filter is employed.
The rate of dioxin removal by bag filters varies widely, i.e., from 80% to 99%, and it is considered possible to lower the dioxin concentration at the bag filter outlet to about 0.5 ng/Nm.sup.3. However, it is necessary in order to lower the dioxin concentration below this level to consider adopting an activated carbon filter or the like.
The activated carbon filter provides a high dioxin removal rate of about 99%, but the SV value (raw gas quantity/activated carbon quantity) is of the order of 400. Accordingly, employment of the activated carbon filter results in a considerable increase in the overall size of the exhaust gas treating apparatus, and a rise in the cost as well as an increase in the installation space.